1. Field of the Invention
The invention relates to a process for synthesizing dispersion polymers and, more particularly, to a process for synthesizing high molecular weight anionic dispersion polymers.
2. Description of the Prior Art
Anionic or nonionic dispersion polymers, which are completely free of oil and emulsifiers, are not available in the market today. Although claims were made in U.S. Pat. No. 3,493,500 issued to Dow Chemical Company in 1970 and Kyoritsu Yuki in U.S. Pat. No. 4,929,655 in 1991 for methods of manufacturing aqueous anionic/nonionic dispersion polymers, stable dispersion polymers could not be made by either of the above processes. Further, the Yuki process requires stabilization of the dispersed particles with an oil and an emulsifier. Because of environmental regulations, there is an increasing demand, for polymers free of oil and surfactants.
In the process of dispersion polymerization, the monomer and the initiator are both soluble in the polymerization medium, but the medium is a poor solvent for the resulting polymer. Accordingly, the reaction mixture is homogeneous at the onset, and the polymerization is initiated in a homogeneous solution. Depending on the solvency of the medium for the resulting oligomers or macroradicals and macromolecules, phase separation occurs at an early stage. This leads to nucleation and the formation of primary particles called "precursors" and the precursors are colloidally stabilized by adsorption of stabilizers. The particles are believed to be swollen by the polymerization medium and/or the monomer, leading to the formation of spherical particles having a size in the region of .about.0.1-10.0 microns.
In any dispersion polymerization, the variables that are usually controlled are the concentrations of the stabilizer, the monomer and the initiator, solvency of the dispersion medium, and the reaction temperature. It has been found that these variables can have a significant effect on the particle size, the molecular weight of the final polymer particles, and the kinetics of the polymerization process.
Particles produced by dispersion polymerization in the absence of any stabilizer are not sufficiently stable and may coagulate after their formation. Addition of a small percentage of a suitable stabilizer to the polymerization mixture produces stable dispersion particles. Particle stabilization in dispersion polymerization is usually referred to as "steric stabilization". Good stabilizers for dispersion polymerization are polymer or oligomer compounds with low solubility in the polymerization medium and moderate affinity for the polymer particles. In many instances, the stabilizer may be grafted on to the surface of the polymer particles, either during or after the polymerization process.
As the stabilizer concentration is increased, the particle size decreases, which implies that the number of nuclei formed increases with increasing stabilizer concentration. The coagulation nucleation theory very well accounts for the observed dependence of the particle size on stabilizer concentration, since the greater the concentration of the stabilizer adsorbed the slower will be the coagulation step. This results in more precursors becoming mature particles, thus reducing the size of dispersions produced.
As the solvency of the dispersion medium increases, (a) the oligomers will grow to a larger MW before they become a precursor nuclei, (b) the anchoring of the stabilizer moiety will probably be reduced and (c) the particle size increases. As the initiator concentration is increased, it has been observed that the final particle size increases. As for the kinetics, it is reported that when the dispersion medium is a non-solvent for the polymer being formed, then the locus of polymerization is largely within the growing particles and the system follows the bulk polymerization kinetics, n (the kinetic chain length)=R.sub.p /P.sub.t, where R.sub.p is the propagation rate and R.sub.t is the termination rate. As the solvency of the dispersion medium for the growing polymer particle is increased, polymer growth proceeds in solution. The polymeric radicals that are formed in solution are then captured by growing particles. Consequently, the locus of the particle polymerization process changes and there is a concomitant change in the kinetics of polymerization.